The present invention relates to the preparation of ceramic chars having increased carbon contents which are derived from methylpolysilanes, and more particularly to a method of preparing such methylpolysilanes by a catalyzed redistribution of alkoxydisilanes, and the methylpolysilanes and ceramic materials produced thereby.
In recent years, workers in the art have developed procedures for the preparation of silicon carbide ceramic materials from polymeric silane precursors such as methylpolysilanes by pyrolyzing the polymers to form ceramic chars. Silicon carbide possesses a number of desirable properties such as chemical inertness, semiconducting properties, extreme hardness and stability at very high temperatures. Accordingly, silicon carbide ceramics have found use in electrical heating units, furnace walls, mufflers, abrasives, rocket nozzles, and automotive and turbine engine parts. Further, it has been found that the use of polymeric precursors permits the formation of fibers and thin films or coatings of silicon carbide which were heretofore extremely difficult to form using inorganic sources of silicon carbide.
However, for many of the above applications to be successful, the composition of the polymer, and the ceramic char derived therefrom, needs to be controlled. Specifically, to provide suitable ceramic materials from such polymers, the carbon content of the polymers, and their resulting chars, must be controlled to increase the carbon content. This provides a ceramic material with a carbon to silicon ratio of closer to 1:1. In some instances it may even be desirable to have a slight excess of carbon. However, not only must the carbon be added to the polymer, but it must be incorporated in such a way that it is not lost from the polymer during pyrolysis to form the ceramic char.
Baney et al, U.S. Pat. No. 4,310,651, teach a procedure for the preparation of methylpolysilanes having halogen substituents through a catalyzed redistribution reaction utilizing tetrabutylphosphonium chloride as the catalyst. Such methylpolysilanes are taught to be useful as ceramic precursors. The Baney et al process has the advantage of being able to utilize as a starting material the process residue from the direct synthesis of organochlorosilanes. Direct synthesis of organochlorosilanes involves passing the vapor of an organic chloride over heated silicon and a catalyst. See, Eaborn, Organosilicon Compounds, Butterworths Scientific Publications, 1960, page 1. This residue contains a mixture of di-, tri-, and tetra-substituted halodisilanes.
However, the halogen substituents on the methylpolysilanes of the Baney et al process have resulted in some difficulties in handling the compositions which tend to auto-ignite when exposed to oxygen or moisture. Moreover, pyrolysis of the compositions releases large quantities of corrosive HCl or HBr gases which must be handled and properly disposed of.
Baney et al, U.S. Pat. No. 4,298,558, teach an improved procedure which converts the halogen substituents on the methylpolysilanes to alkoxy or phenoxy substituents. Baney et al, U.S. Pat. No. 4,298,559, teach a procedure which converts the halogen substituents on the methylpolysilanes to alkyl or phenyl substituents. However, the improved procedures still require a two step process of converting halodisilanes to halo-substituted methylpolysilanes and then converting the halogen substituents to alkoxy, phenoxy, alkyl, or phenyl substituted compositions.
Haluska, U.S. Pat. No. 4,546,163, teaches a procedure for forming vinyl-containing polysilanes by reacting alkyl halodisilanes with a halosilane containing a vinyl group. The resulting polymers are taught to be useful as ceramic precursors. Again, however, halogen-containing starting materials must be utilized.
Other workers have attempted to produce methylpolysilanes by a single step redistribution reaction using methoxydisilane starting materials. For example, Ryan et al, 84 J. Amer.Chem.Soc. 4730 (1962), reported the redistribution of 1,1,2,2-tetramethoxy-1,2-dimethyldisilane to higher polysilanes in the presence of sodium metal. Watanabe et al, in a series of published reports, taught that metal alkoxide catalysts could be used in the redistribution reaction. See, e.g., Watanabe et al, J. C. S. Chem. Comm. (1977) 534; Watanabe et al, J. C. S. Chem. Comm. (1977) 704; Watanabe et al, 128 J. Organometallic Chem. 173 (1977); Watanabe et al, J. C. S. Chem. Comm., (1978) 1029; Watanabe et al, 218 J. Organometallic Chem. 27 (1981); and Watanabe et al, 244 J. Organometallic Chem. 329 (1983).
Atwell et al, 7 J. Organometallic Chem. 71 (1967), have also reported the redistribution of alkoxy disilanes to higher organopolysilanes. However, in the Watanabe and Atwell reports, the higher organopolysilane was either uncharacterized, unidentified, or was of a low molecular weight (less than 6 silicon atoms in the chain).
More recently, Frey et al, U.S. Pat. No. 4,667,046, teach a method for preparing higher molecular weight methylpolysilanes by reacting a trialkoxy-substituted disilane, and optionally a tetraalkoxy-substituted disilane, with a silane having at least one silicon to hydrogen bond in the presence of an alkali metal alkoxide catalyst. The methylpolysilanes are taught to be useful as negative photoresist coatings and ceramic precursors.
However, the prior art does not teach a process for controlling or increasing the carbon content of ceramic precursor polymers and maintaining that additional carbon content in the ceramic char. Accordingly, the need still exists in the art for a process for the preparation of ceramic precursor polymers which have increased carbon contents in both the polymer and the resultant ceramic char.